During the fabrication of rubber articles, it is common for unvulcanized calendered sheets to be laminated to obtain the desired structural configuration. This technique of building up of layers of uncured elastomer is used extensively in the tire building industry, but also finds utility in the manufacture of other rubber articles such as mechanical goods and hoses. In order that the unvulcanized composites have the necessary mechanical stability toward handling and storage, the elastomeric materials must have sufficient tack so that the desired configuration is retained through the vulcanization step. This tack is the ability of unvulcanized elastomer to adhere to itself or to another elastomer which also has tack. This adhesive property, known as "building tack" plays an important role in the production of rubber goods. In the manufacture of tires, "building tack" holds the innerliner, beads, plys, sidewalls and tread together prior to vulcanization.
Natural rubber has the advantage of having sufficient tack without tackifying resins being added. However, synthetic rubbers lack this building tack, and the use of resinous modifiers is necessary to obtain the tack required for fabrication. It is not uncommon, however, to add tackifiers to natural rubber or to blends of natural and synthetic compounds to aid in tack retention during storage of calendered unvulcanized stock. Tackifiers also provide secondary benefits by reducing compound viscosity and also by functioning as plasticisers.
A number of different types of materials are utilized as rubber tackifiers. These include various hydrocarbon resins made from feedstocks derived principally from petroleum cracking and coal tar operations. Aliphatic and aromatic type hydrocarbon resins are produced primarily from petroleum derived streams. While coumarone-indene resins are coal tar derived, similar resins are also made from petroleum sources. Polyterpenes, terpene phenolics, rosin and rosin derivatives, alkylphenol-aldehyde resins, alkylphenol acetylene resins, natural rubber and reclaimed rubber are also utilized as rubber tackifiers.
The hydrocarbon resins are in general less expensive than are the phenolics, but require the use of up to three times the amount to give equivalent tack with tack retention being adversely affected. Since tackifiers which remain in the fabricated rubber article can tend to detract from the properties of the rubber, phenolics are often advantageously used because of the lower level required. However, phenolic tackifiers do tend to decrease the adhesion of the rubber to wire or other reinforcing fibers. For this reason, hydrocarbon resins are sometimes used, even though higher percentages are required, when adhesion of the rubber to reinforcing materials is critical. This invention provides phenolic tackifiers having desirable efficiency with no adverse effect on the adhesion of the rubber to the reinforcing element.